Heat-resistant resins, which have excellent mechanical and chemical properties, have been molded into many different shapes such as films, tubes, rods, formed materials, coating materials, etc., and used as flexible printed substrates, heat-resistant electric wire insulating materials, magnetic tapes, and the like. Future additional uses of the heat-resistant resins are also likely to become apparent. The heat resistant resins are used typically as revolving and delivering belts or conveyor belts for light electrical sounders, or heat fixing belts for copiers, laser beam printers, and the like.
As a heat-fixing member of a fixing apparatus in order to fix toner developed on copying paper or decalcomania paper, a heat roller is generally used in copiers or laser beam printers that utilize electrophotographic technology. More specifically, the papers developed with toner are passed through an opening between a fixing roller having a heating mechanism (heat roller) and a pressure roller, one page following another, thereby heating, melting and then fixing the toner on the paper.
There has been research on the use of a polyimide tube, instead of the heat roller, for a fixing apparatus. The inside polyimide tube is equipped with a live roller, a tension roller and a heater. Copying paper developed with toner is supplied one after another to an opening between the polyimide tube and a backing-up roller, thus fixing toner on the paper. This fixing apparatus heats and fixes the toner at the surface of the polyimide tube via the heater. Therefore, different from the heat roller, this polyimide tube does not require time for heating itself, and can start fixing toner on paper as soon as the power supply switch of the fixing apparatus is turned on. In addition, the capacity of the heater used for this fixing apparatus is small, and the apparatus consumes little electricity.
When this fixing apparatus is used, the toner developed on copying paper is instantaneously melted and fixed on the paper by the heater employed inside the polyimide tube. Therefore, if the polyimide tube wall thickness is uneven, the toner cannot be melted evenly. As a result, an undesirable offset phenomenon occurs. In this sense, it is necessary to minimize any uneveness in the polyimide tube wall thickness much as possible.
When the polyimide tube having uneven internal diameters in a longitudinal direction is rotated by two or three rollers, the tube meanders in a longitudinal direction. Therefore, when the tube is used as a heat-fixing seamless belt, the tube is required to have precise cylindricity.
One example of a method of manufacturing polyimide tubes with uniform tube wall thickness is disclosed in Japanese Published Unexamined Patent Application No. Sho 62-19437. The polyimide tube is manufactured in the following steps:
pouring polyamic acid solution into a molding pipe such as a glass pipe, stainless pipe, or the like with a smooth internal surface; PA1 holding the molding tube in a vertical position; PA1 dropping a bullet-like object through the solution by its own weight, thereby forming a hole inside the solution; PA1 heating and drying the solution inside the molding pipe, thus causing it to become imide by imide reaction and forming a tube; and PA1 extracting the tube from the molding pipe. PA1 coating a polyimide precursor solution on the outside surface of a metallic cylinder; PA1 casting the precursor solution with a metallic ring at a uniform thickness; PA1 drying and heating the solution, thus forming a half-hard polyimide layer by a midway imide reaction; PA1 coating a conductive primer layer on the surface of the half-hard polyimide layer; PA1 coating a fluororesin layer on the surface of the conductive primer layer; and PA1 heating the half-hard polyimide layer coated with the conductive primer layer and the fluororesin layer to complete the imide reaction as well as to bake the fluororesin layer at the same time. PA1 coating a polyimide precursor solution on the outside surface of a metallic cylinder; PA1 casting the precursor solution with a metallic ring at a uniform thickness; PA1 drying and heating the solution, thus forming a half-hard polyimide layer by a midway imide reaction; PA1 coating a conductive primer layer on the surface of the half-hard polyimide layer; PA1 coating a fluororesin layer on the surface of the conductive primer layer; and PA1 heating the half-hard polyimide layer coated with the primer layer and the fluororesin layer to complete the imide reaction as well as to bake the fluororesin layer at the same time. PA1 wherein x represents reduction ratio of the thickness of the polyimide precursor layer; PA1 V.sub.o represents the original thickness of the polyimide precursor solution right after the solution is coated on the metallic cylinder; and PA1 V.sub.a represents the thickness of the half-hard polyimide layer at the midway imide reaction. PA1 a component for casting a polyimide precursor solution coated on the surface of a metallic cylinder at a uniform thickness; PA1 a component for heating the polyimide precursor solution to make it dry or half-hard; PA1 a component for coating conductive primer on the surface of the half-hard polyimide precursor layer; PA1 a component for coating fluororesin; and PA1 a heating component to complete the imide reaction and to bake the fluororesin at the same time.
The inventors of the present invention also disclose another method in Japanese Published Unexamined Patent Applications No. Hei 3-180309 and No. 3-261518. In this method, a polyimide precursor solution such as polyamic acid solution is coated on the outside surface of a core. The solution on the core is then heated and dried, thus causing it to become imide by imide reaction and forming a tube. Finally, the tube is separated from the core.
However, since only an extremely thin tube can be formed by the method of Japanese Published Unexamined Patent Application Sho 62-19437, the tube has to be laminated repeatedly by repeating the forming, drying and heating steps of this method many times. It is also extremely difficult to extract the tube from the inside surface of the glass or stainless pipe. Since the polyimide tube is extracted from the inside of the pipe, a long polyimide tube with a small inside diameter can hardly be made. Moreover, when the polyimide tube is used as a fixing means for an electrophotographic printer or a laser printer to fix toner developed on copying papers at the tube surface via the heater, the tube's properties of separating the toner from itself are so critical that offset occurs. In other words, toner left on the tube is later printed on copying paper by the rotation of the tube, thus staining both the paper and the tube surface. Since the polyimide tube is also likely to generate static electricity, the toner, right before its fixation, is repulsed by the static, thereby blurring copy image and weakening resolution.
Similar to the above-noted problems, Japanese Published Unexamined Patent Applications No. Hei 3-180309 and No. Hei 3-261518 have problems of poor separation properties as well as static electricity of the polyimide tubes.